Power requirements and size are critical design parameters in portable electronic devices such as pagers and broadcast frequency modulation (FM) radio receivers. In an increasingly energy conscious world, these two design parameters are also important in other electronic devices that operate when plugged into an alternating current (AC) supply.
A circuit that is useful within integrated circuits in such devices is a current reference circuit. Such a circuit is particularly useful in analog computation circuits, particularly when variables being used for computation are expressed as a current, a ratio of currents, or as related to a bias current. It is commonplace to provide a circuit which acts as a reference by starting up automatically and maintaining a substantially constant current, even though a power source supplying the current has a voltage that varies. Typical of such circuits are feedback circuits that depend on a gain non-linearity to stabilize at a reference current. The non-linearity establishes a loop gain that is greater than one for small currents and less than one for larger currents. The feedback circuit then reaches a steady state reference value where the loop gain is one. In current mirror circuits, a resistor is typically used to achieve this type of gain non-linearity in one of a feed forward current mirror or feedback current mirror in the feedback circuit. This is accomplished by using the resistor to alter the current-versus-voltage performance (wherein the voltage is a gate to source voltage of a bias transistor) of either a mirror input or output stage in one of the feed forward or feedback current mirrors. The transistors in the input and output stages in a typical current mirror have non-linear diode current-versus-voltage characteristics that are very well matched, and the typical current mirror thereby provides a gain that is essentially constant over an operational voltage range. The resistor modifies the diode characteristics of one or more transistors in one of the input or output stages of either or both of the feed forward and feedback current mirrors, thus providing the desired non-linear gain characteristic that includes a unity gain. However, using a resistor has a drawback in that they are physically very large compared to the size of transistors in modern day integrated circuits. The physical size of such a resistor can easily exceed the size of the all the transistors used in such a feedback reference circuit, and there are also difficulties in matching to a particular transistor characteristic in order to achieve the desired non-linear gain, and in trying to minimize their variation over temperature changes.
Thus, what is needed is a current reference circuit that is small, requires little power to operate, and has no resistors.